This invention relates generally to cathode ray tube (CRT) burn protection circuits and specifically to a burn protection circuit for use with a projection type CRT.
Burn protection circuits for CRTs are well known. They are designed to prevent the occurrence of a relatively stationary high energy electron beam impinging on the phosphor covered faceplate of the CRT. In color CRTs of the shadow mask variety, the shadow mask absorbs a substantial portion of the electron beam energy and burning of the CRT phosphor is not considered a severe problem. Monochrome CRTs and projection type CRTs, on the other hand, do not have shadow masks and there is an increased potential for dangerously high concentrations of beam energy on areas of the CRT. The problem is generally caused by the collapse of the CRT electron beam deflection system before the high voltage system has been shut down and its energy dissipated. The above situation can result in a relatively stationary high energy electron beam impacting a small, central area of a the CRT screen.
In many television and monitor applications, the deflection system and the high voltage generating system are interrelated, that is, the high voltage is developed by rectification of a so-called flyback pulse that is developed in the deflection system. Other systems have a deflection circuit that is independent of the high voltage generating circuit. In either system, current art prescribes a shutdown sequence for assuring that, under normal operating conditions, high voltage generation is disabled before the deflection circuit is disabled. In certain microprocessor based television receivers and projection systems, a special high voltage shutdown transistor may be actuated by a suitable signal to rapidly disable the high voltage generating system. Also, a separate so called "blank" transistor may be energized for driving the CRT into cutoff. Such systems are well known in the art.
The present invention is especially useful in a projection monitor that includes independent high voltage and deflection circuits, for preventing CRT phosphor burn conditions from arising when the AC line power is interrupted during monitor operation.